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Computers containing PFC (Power Factor Corrected) power supplies and their use Back-UPS and Smart-UPS SC with Step Approximated Sine waves.

Issue:

Power Factor Corrected (PFC) power supplies are becoming increasingly popular in desktop PC’s in North America. Contributing to this trend is the fact that Energy Star 4.0 compliance as of July 2007, requires the use of PFC in desktop PC’s.

Please see reference to the Energy Star 4.0 compliance Tier 1 at www.energystar.gov/

What is Power Factor and Power Factor Correction?
Power factor is the percentage of electricity that is being used to do useful work. It is expressed as a ratio. For example, a power factor of 0.72 would mean only 72% of your power was being used to do useful work. Perfect power factor (which in this case is being achieved by the computer’s PFC power supply) is 1.0 (unity), meaning 100% of the power is being used for useful work. Power Factor Correction is a circuit design technique to increase the power factor of a device so that it approaches 1, or unity power factor.

Although computer power supplies draw only a fraction of their full capacity during it’s steady state(normal operation), PFC power supplies have the potential to draw their full capability during initial inrush. ""Inrush"" or ""Inrush Current"" refers to the maximum instantaneous input current drawn by an electrical device when first turned on.

Resolution:

A computer’s power supply may also be subjected to a period of inrush, while the UPS is changing state (switching from utility power to battery power and back). Back-UPS and Smart-UPS SCs may experience up to an 8ms transfer time during this period. This is just long enough to remove power from the PFC power supply, resulting in a momentary inrush of the PFC. Once the UPS changes states from ""Online"" (passing utility power) to ""Onbattery"" (passing power from the UPS's internal battery), the momentary inrush from the attached equipment subjects the UPS to the On battery power supply’s maximum power draw, resulting in a potential Overload condition or dropped load.

An Energy Star 4.0 compliant power supply has to be more than 80% efficient. For example, if a attached power supply is delivering 600W output power, its ‘input’ power can be as high as 750W. .

This ‘input’ power should be the basis for sizing the UPS, so as not to Overload the UPS. This can be calculated by taking the PFC power supply’s rated output power and multiplying it by 1.25 as follows;